Internal part feature cutting method and apparatus

ABSTRACT

A method for cutting an internal feature in a workpiece using a plasma cutting system can include cutting in a first zone using at least one cutting parameter from a first cutting parameter set, which can include a first cutting current and/or a first speed. The method can include cutting in a second zone using at least one cutting parameter from a second cutting parameter set that can be different from the first cutting parameter set. The second cutting parameter set can include a second cutting current and/or a second speed. The method can also include cutting in a third zone using at least one cutting parameter from a third cutting parameter set, which can be different from the first cutting parameter set or the second cutting parameter set. The third cutting parameter set can include a third cutting current and/or a third speed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 61/154,259 filed on Feb. 20, 2009, which is ownedby the assignee of the instant application and the disclosure of whichis incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to plasma arc cutting torch systems.More specifically, the invention relates to a method and apparatus forcutting internal features and contours in a workpiece using a plasmatorch tip configuration.

BACKGROUND OF THE INVENTION

Plasma cutting uses a constricted electric arc to heat a gas flow to theplasma state. The energy from the high temperature plasma flow locallymelts the workpiece. FIG. 1 is a diagram of a known automated plasmatorch system. Automated torch system 10 can include a cutting table 22and torch 24. An example of a torch that can be used in an automatedsystem is the HPR260 auto gas system, manufactured by Hypertherm, Inc.,of Hanover, N.H. The torch height controller 18 can be mounted to agantry 26. The automated system 10 can also include a drive system 20.The torch is powered by a power supply 14. The plasma arc torch systemcan also include a gas console 16 that can be used to regulate/configurethe gas composition (e.g., gas types for the shield gas and plasma gas)and the gas flow rates for the plasma arc torch. An automated torchsystem 10 can also include a computer numeric controller 12 (CNC), forexample, a Hypertherm Automation Voyager, manufactured by Hypertherm,Inc., Hanover, N.H. The CNC 12 can include a display screen 13 which isused by the torch operator to input or read information that the CNC 12uses to determine operating parameters. In some embodiments, operatingparameters can include cut speed, torch height, and plasma and shieldgas composition. The display screen 13 can also be used by the operatorto manually input operating parameters. A torch 24 can also include atorch body (not shown) and torch consumables that are mounted to thefront end of a torch body. Further discussion of CNC 12 configurationcan be found in U.S. Patent Publication No. 2006/0108333, assigned toHypertherm, Inc., the disclosure of which is incorporated herein byreference in its entirety.

FIG. 2 is a cross-sectional view of a known plasma arc torch tipconfiguration, including consumable parts and gas flows. The electrode27, nozzle 28, and shield 29 are nested together such that the plasmagas 30 flows between the exterior of the electrode and the interiorsurface of the nozzle. A plasma chamber 32 is defined between theelectrode 27 and nozzle 28. A plasma arc 31 is formed in the plasmachamber 32. The plasma arc 31 exits the torch tip through a plasmanozzle orifice 33 in the front end of the nozzle to cut the workpiece37. The shield gas 34 flows between the exterior surface of the nozzleand the interior surface of the shield. The shield gas 34 exits thetorch tip through the shield exit orifice 35 in the front end of theshield, and can be configured to surround the plasma arc. In someinstances, the shield gas also exits the torch tip through bleed holes36 disposed within the shield 29. An example of plasma torch consumablesare the consumable parts manufactured by Hypertherm, Inc., of Hanover,N.H. for HPR 130 systems, for cutting mild steel with a current of 80amps. The nozzle 28 can be a vented nozzle, (e.g., comprising an innerand outer nozzle piece and a bypass channel formed between the inner andouter nozzle pieces directs the bypass flow to atmosphere), as describedin U.S. Pat. No. 5,317,126 entitled “Nozzle And Method Of Operation ForA Plasma Arc Torch” issued to Couch et al., which is owned by theassignee of the instant application and the disclosure of which isincorporated herein by reference in its entirety.

Internal features (e.g., hole features, substantially circular holes,slots, etc.) cut with plasma arc torches using known methods can resultin defects, such as, for example, protrusions, divots, “bevel” or“taper.” Bevel or taper is where a feature size at a bottom side of theworkpiece is smaller than the feature size at the top side of the plate.For example, the diameter of an internal feature such as a hole feature,at the top of the workpiece should be cut to match the size of a bolt topass through the hole feature. If the hole feature has defects, such as,protrusions, divots, bevel or taper, the defects in the hole feature cancause the hole feature diameter to vary from the top of a workpiece tothe bottom of the workpiece. Such defects can prevent the bolt frompassing through the bottom of the workpiece. Secondary processes, suchare reaming or drilling are required to enlarge the diameter of the bolthole feature at the bottom of the workpiece. This prior method ofensuring hole cut quality can be time consuming, suggesting that a moreefficient method of cutting holes and contours in a single workpiece isneeded.

The assignor of this patent (Hypertherm, Inc.) conducted extensivecustomer research into the areas of plasma cutting that requireimprovement. During the research, 22 end users out of a total 45 endusers provided the unsolicited request that improving the quality ofhole features is an area of plasma cutting that they would most like tosee improved. Improving the cut quality of hole features was mentionedover 3 times more than the next response. This feedback from the endusers demonstrates that quality plasma hole cutting is perceived by theend users continues to be a long-felt unmet need.

Traditionally, to correct defects in the hole feature, such as, forexample, a “protrusion” where the lead-in of a cut transitions into aperimeter of the cut, the arc is left on after cutting the perimeter to“clean up” the defect left by the lead-in by cutting the unwanted excessmaterial. This process is called “over burn.” Over burning, however, canresult in removing too much material, leaving an even larger defect(e.g., leaving a divot in place of the protrusion).

SUMMARY OF THE INVENTION

The present invention substantially improves the cut quality for smallinternal part features (e.g., hole features) cut from a workpiece usinga plasma arc torch. Hole features cut using plasma arc torches usingknown methods can result in features with defects, such as protrusions(e.g., where not enough material was cut), divots (e.g., where too muchmaterial was cut), bevel or taper, which can prevent, for example, abolt from passing through the bottom of a workpiece. Cutting parameters(e.g., gas composition, cutting speed, cutting current, etc.) can bemanipulated to improve the cut quality of a small internal part feature.For example, the cutting speed of the “lead-in” of the cut can affectthe cut quality for the hole feature. Using the same speed for thelead-in of the cut as the rest of the cut can result in defects such asprotrusions, where the lead of the cut transitions into the perimeter.As noted above, traditionally, an “over burn” process can be used toremove the excess material, however, an over burn process can remove toomuch material, leaving behind a defect such as a divot. Using a low N₂gas composition (e.g., a gas composition of O₂ plasma gas and O₂ shieldgas) can also be used to cut small internal features to help minimizethe bevel and/or taper of the feature. Such a method of cutting isdisclosed in U.S. patent application Ser. No. 12/341,731, entitled “HighQuality Hole Cutting Using Variable Shield Gas Compositions” filed onDec. 22, 2008, which is owned by the assignee of the instant applicationand the disclosure of which is incorporated herein by reference in itsentirety. Cutting a workpiece using, for example air, is not assensitive to defects as cutting with O₂ gas. Using O₂ plasma and O₂shield gas can further amplify defects such as protrusions and/or divotsin the hole feature, suggesting that a method of reducing defects isneeded. While laser cutting systems can yield high quality cuts, plasmaarc torch systems provide a low cost alternative to cutting internalfeatures (e.g., hole features). Therefore, there is a need for highquality internal features cut from plasma arc torch systems.

The bevel can be measured by the cylindricity of the completed hole cut.Cylindricity is defined as a tolerance zone that is established by twoconcentric cylinders between which the surface of a cylindrical holemust lie as illustrated in FIG. 3A. In FIG. 3A the tolerance zone can bedefined as the space between the two arrows 81. The smaller thetolerance zone, the more the surface represents a “perfect” cylinder. Alarge taper or bevel in a hole feature, on the other hand, will resultin a large tolerance zone. Cylindricity of a hole feature can also bemeasured using a coordinate-measuring machine (“CMM”). For example, thehole feature surface (e.g., encompassing taper, protrusions, or divots)of the hole feature can be measured near of the top 71, middle 72, andbottom 73 of the edge 74 of the hole feature. This measurement data isused to form concentric cylinders defining the cylindricity of the holefeature. The radial difference between the concentric cylinders isillustrated by the space between the arrows 81. The large differencebetween the radiuses of the two datum cylinders in FIG. 3B indicates apoor quality hole feature. Such holes can require significant postcutting treatment.

A hole feature can be defined as a shape having a diameter (ordimension) to workpiece (plate) thickness ratio of approximately 2.5 orsmaller. By way of example, a 1 in diameter hole in a 0.5 inch thickplate of steel would have a ratio of 2. A hole feature, as used herein,can be categorized as a small internal part feature that is notnecessarily round, but where a majority of the features have dimensionthat are about 2.5 times or less than the thickness of the materials(e.g., a 1 in square in the ½ inch plate steel).

In one aspect, the invention features a method for cutting an internalfeature, such as a hole feature, in a workpiece using a plasma arctorch. The plasma arc torch can be used to cut along a portion of a pathincluding a first zone, a second zone, and a third zone using a plasmacutting system. The method can include cutting in the first zone usingat least one cutting parameter from a first cutting parameter set, thefirst cutting parameter set including a first cutting current and/or afirst command speed establishing a first torch speed. The method canalso include cutting in the second zone using at least one cuttingparameter from a second cutting parameter set. The second cuttingparameter set can be different from (e.g., where at least one parameterin the set is different) the first cutting parameter set and can includea second cutting current and/or a second command speed establishing asecond torch speed. The method can also include cutting in the thirdzone using at least one cutting parameter from a third cutting parameterset. The third cutting parameter set can be different from the firstcutting parameter set or the second cutting parameter set and caninclude a third cutting current and/or a third command speedestablishing a third torch speed.

The command speed can be a set point for a torch/cutting speed. Thetorch speed can be the command speed offset by anacceleration/deceleration of the torch to reach the command speedsetpoint and inefficiencies/limitations inherent in the plasma arc torchsystem.

In some embodiments, the first zone corresponds to a lead-in of a cut,the second zone corresponds to a perimeter of the cut, and the thirdzone corresponds to a kerf break-in region of the cut. The hole featurecan be defined, at least in part, by an outer kerf edge of a cut in thesecond zone and at least a portion of an outer kerf edge of a cut in thethird zone. Cutting in the first zone can include cutting a semi-circlein the workpiece.

The second command speed can be greater than the first command speed. Insome embodiments, the third cutting current is less than the secondcutting current during at least a portion of the third zone.

In one aspect, the invention features a method for cutting an internalfeature, such as a hole feature, in a workpiece along at least a portionof a path including a first zone and a second zone using a plasmacutting system. The method can include the steps of initiating a plasmagas flow, initiating a current flow to ignite a pilot arc, transferringthe arc to the workpiece and piercing the workpiece (e.g., to begincutting an internal feature in the workpiece). The method can includecutting in the first zone using a first command speed establishing afirst torch speed and cutting in the second zone using a second commandspeed establishing a second torch speed. The second command speed can begreater than the first command speed.

The internal feature (e.g., hole feature) can be a substantiallycircular hole or a slot.

The path can include a third zone. The first zone can correspond to alead-in of a cut, the second zone can correspond to a perimeter of thecut, and the third zone can correspond to a kerf break-in region of thecut. The method can also include ramping down a cutting current in thethird zone such that the cutting current reaching substantially zeroamperes at a location corresponding to a beginning of the second zonewhere the first zone, second zone and third zone substantiallyintersect. The cutting current can be ramped down at a rate based, atleast in part, upon a length between a beginning of the third zone andthe beginning of the second zone.

The first command speed can be based at least in part on a diameter ofthe hole feature. The torch speed can be reduced after the cuttingcurrent reaches substantially zero amperes. A third command speed can beused, for example, to cut in the third zone. The third command speed candefine a third torch speed. A ramp down of the cutting current can beinitiated at a location in the third zone determined by the third torchspeed and a ramp down time (e.g., the time required for the current toreach substantially zero amperes).

The method can include cutting in the first zone or in the second zoneusing a gas flow composition comprising O2 plasma gas and O2 shield gas.

In another aspect, the invention features a method for cutting aninternal feature (e.g., a hole feature) in a workpiece along at least aportion of a path including a first zone, a second zone, and a thirdzone using a plasma cutting system. The method can include the steps ofinitiating a plasma gas flow, initiating a current flow to ignite apilot arc, transferring the arc to the workpiece and piercing theworkpiece (e.g., to begin cutting an internal feature/hole feature inthe workpiece). The method can include cutting in a first zone and asecond zone. The command speed of a cut for the second zone can bedifferent than a command speed of a cut in the first zone. The methodcan also include reducing a cutting current in the third zone such thatthe cutting current reaches substantially zero amperes at a point wherean outer kerf edge of a cut in the third zone substantially meets anouter kerf edge of the cut in the first zone. The method can alsoinclude decelerating a torch speed of the plasma cutting system afterthe cutting current has reached substantially zero amperes.

In some embodiments, the method can include cutting in the second zonewith a command speed greater than the command speed of the cut in thefirst zone.

A distance from a center of the hole feature to an outer kerf edge ofthe cut in the second zone can be substantially similar to a distancefrom the center of the hole feature to an outer kerf edge of the cut inthe third zone at a point where the first and third zone intersect. Thehole feature can be substantially defined by an outer kerf edge of thecut in the second zone and at least a portion of the outer kerf edge ofthe cut in the third zone. In some embodiments, the first zonecorresponds to a lead-in of the cut, the second zone corresponds to aperimeter of the cut and the third zone corresponds to a kerf break-inregion of the cut.

The torch speed can be decelerated after a point where the outer kerfedge of the cut in the first zone substantially intersects with theouter kerf edge of the cut in the third zone. The torch speed can bedecelerated to reach zero at a predetermined distance after the pointwhere the outer kerf edge of the cut in the first zone substantiallyintersects with the outer kerf edge of the cut in the third zone.

In some embodiments, the cutting current in the third zone can be rampeddown such that the cutting current reaches substantially zero amperes ata location where an outer kerf edge of the cut in the first zonesubstantially intersects with an outer kerf edge of the cut in the thirdzone.

In another aspect, the invention features a method for cutting aninternal feature (e.g., a hole feature) in a workpiece along at least aportion of a path including a first zone, a second zone, and a thirdzone and using a plasma cutting system. The method can include the stepsof initiating a plasma gas flow, initiating a current flow to ignite apilot arc, transferring the arc to the workpiece and piercing theworkpiece (e.g., to begin cutting an internal feature/hole feature inthe workpiece). The method can include cutting in the second zone with acommand speed different from a command speed of the first zone of thecut. The method can also include ramping down a cutting current in thethird zone to remove a diminishing material such that an outer kerf edgeof a cut in the third zone substantially aligns with an outer kerf edgeof a cut in the second zone. The method can include decelerating a torchspeed of the plasma cutting system after the cutting current has reachedsubstantially zero amperes.

In some embodiments, the cutting current can be ramped down in the thirdzone so that the cutting current reaches substantially zero ampereswhere the outer kerf edge of the cut in the third zone intersects withthe outer kerf edge of a cut in the first zone. The diminishing materialcan be defined at least in part by an outer kerf edge of the cut in thefirst zone and an outer kerf edge of the cut in the third zone. In someembodiments, the third zone can be cut with a command speed greater thanthe command speed of the second zone of the cut.

In yet another aspect, the invention features a method for cutting aninternal feature (e.g., a hole feature) in a workpiece using a plasmacutting system that reduces defects in the hole feature. The method caninclude the steps of initiating a plasma gas flow, initiating a currentflow to ignite a pilot arc, transferring the arc to the workpiece andpiercing the workpiece at the beginning of a cut (e.g., to cut aninternal feature/hole feature in the workpiece). The method can includeestablishing a cutting arc and a cut speed with respect to the workpieceand increasing the cut speed to a second cut speed after a first pointin a hole cut path. The method can also include ramping down a cuttingcurrent after a second point in the hole cut path without reducing thecut speed. The second cut speed can be maintained until the cuttingcurrent reaches substantially zero amperes. The second cut speed canalso be increased to a third cut speed before the cutting currentreaches substantially zero amperes.

The first zone can define a lead-in of a cut and the second zone candefine at least a portion of a perimeter of the hole feature.

In some embodiments, the step of increasing the cut speed can includecutting in a first zone of the hole cut path with a first command speedand cutting in a second zone of the hole cut path with a second commandspeed greater than the first command speed. The first command speed canbe based on a diameter of the hole feature.

The cutting current can be reduced (i.e., ramp down of the cuttingcurrent initiated) after the second point in the hole cut path and thecutting arc can be extinguished substantially near the first point inthe hole cut path. In some embodiments, the torch can cut from thesecond point in the hole cut path and return back to the first point inthe hole cut path to form the hole feature in the workpiece. The cuttingcurrent can be ramped down while cutting from the second point in thehole cut path back to the first point in the hole cut path.

In another aspect, the invention features a plasma arc torch system forcutting an internal feature (e.g., a hole feature) in a workpiece alongat least a portion of a path including a first zone, a second zone, anda third zone. The plasma arc torch system can include a plasma torchincluding an electrode and a nozzle, a lead that provides a cuttingcurrent to the plasma arc torch, a gantry attached to the plasma torchthat moves the plasma torch and a computer numerical controller thatcontrols cutting parameters of the plasma arc torch in the first zone,the second zone, and the third zone. The computer numerical controllercan establish a first command speed for the first zone and a secondcommand speed for the second zone. The first command speed can be based,at least part, on a diameter of the hole feature. The second commandspeed can be greater than the first command speed. The computernumerical controller can also establish a third cutting current for thethird zone. The third cutting current can ramp down so that the thirdcutting current reaches substantially zero amperes where an outer kerfedge of a cut in the first zone substantially intersects with an outerkerf edge of a cut in the third zone.

The computer numerical controller can include a look-up table toidentify the cutting parameters of the plasma arc torch.

The third cutting current can ramp down to remove a diminishing material(e.g., the remaining material of the workpiece to finish cutting thehole feature) so that an outer kerf edge of the cut in the third zonesubstantially aligns with an outer kerf edge of a cut in the secondzone.

In yet another aspect, the invention features a computer readableproduct, tangibly embodied on an information carrier, and operable on acomputer numeric controller for a plasma arc torch cutting system. Thecomputer readable product can include instructions being operable tocause the computer numeric controller to select cutting parameters forcutting an internal feature (e.g., a hole feature) in a workpiece alongat least a portion of a path comprising a first zone, a second zone, anda third zone. The cutting parameters can include a first command speedfor the first zone based, at least part, on a diameter of the holefeature and a second command speed for the second zone of the cut. Thesecond command speed can be greater than the first command speed. Thecutting parameters can also include a third cutting current for thethird zone, where the third cutting current ramps down such that thethird cutting current reaches substantially zero amperes where an outerkerf edge of a cut in the first zone substantially intersects with anouter kerf edge of a cut in the third zone.

The third cutting current can ramp down to remove a diminishing material(e.g., the remaining material of the workpiece to finish cutting thehole feature) so that an outer kerf edge of the cut in the third zonesubstantially aligns with an outer kerf edge of a cut in the secondzone.

In another aspect, the invention features a method for cutting aninternal feature (e.g., a hole feature) in a workpiece using a plasmaarc torch to reduce defects in the hole feature. The plasma arc torchcan cut along at least a portion of a path including a first zone, asecond zone and a third zone. The method can include selecting one of aplurality of cutting current ramp down operations for cutting in thethird zone, where each of the plurality of cutting current ramp downoperations is a function of a diameter of the hole feature. The methodcan also include extinguishing the plasma cutting current when a torchhead passes from the third zone to the second zone at a location wherethe first zone, second zone and third zone substantially intersect. Themethod can also include substantially maintaining or increasing a torchspeed in the third zone until the torch head passes from the third zoneinto the second zone.

In another aspect, the invention features a method for cutting aninternal feature (e.g., a hole feature) in a workpiece using a plasmaarc torch to reduce defects in the hole feature. The plasma arc torchcan cut along at least a portion of a path including a first zone, asecond zone and a third zone. The method can include the steps ofinitiating a plasma gas flow, initiating a current flow to ignite apilot arc, transferring the arc to the workpiece and piercing theworkpiece (e.g., to begin cutting an internal feature/hole feature inthe workpiece). The method can include cutting alone the first zone andthe second zone of the path. A ramp down of a cutting current can beinitiated at a first point in the third zone such that the cuttingcurrent is extinguished at a second point where the first zone, thesecond zone and the third zone substantially intersect. The first pointin the third zone can be determined based on a ramp down time of thecutting current. The method can also include the step of deceleratingthe plasma arc torch so that a torch speed reaches substantially zero ata predetermined distance after the second point.

In some embodiments, the predetermined distance is about ¼ of an inch.In some embodiments, the predetermined distance is about ¼ of an inchwhere an upper limit of the hole cutting speed is about 55 ipm and alower limit for a table acceleration is about 5 mG.

In another aspect, the invention features a method for cutting a holefeature in a workpiece along at least a portion of a path using a plasmacutting system. The path can include a first zone and a second zone. Themethod can include the steps of initiating a plasma gas flow, initiatinga current flow to ignite a pilot arc, transferring the arc to theworkpiece and piercing the workpiece (e.g., to begin cutting an internalfeature/hole feature in the workpiece). The method can include cuttingin the first zone using a first command speed (e.g., establishing afirst torch speed), where the first command speed is part of anacceleration curve (e.g., programmed for a plasma arc torch system). Themethod can include cutting in the second zone using a second commandspeed (e.g., establishing a second torch speed), where the secondcommand speed is part of the acceleration curve and is also greater thanthe first command speed.

In yet another aspect, the invention features a method for cutting ahole feature in a workpiece along at least a portion of a path using aplasma cutting system. The path can include a first zone and a secondzone. The method can include the steps of initiating a plasma gas flow,initiating a current flow to ignite a pilot arc, transferring the arc tothe workpiece and piercing the workpiece (e.g., to begin cutting aninternal feature/hole feature in the workpiece). The method can includecutting where the first zone and the second zone substantially intersect(e.g., where the lead-in of the cut transitions into the perimeter ofthe cut) at a first torch speed. The method can also include cutting atleast a portion of the second zone at a second torch speed, where thesecond torch speed is greater than the first torch speed.

Other aspects and advantages of the invention can become apparent fromthe following drawings and description, all of which illustrate theprinciples of the invention, by way of example only.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the invention described above, together with furtheradvantages, may be better understood by referring to the followingdescription taken in conjunction with the accompanying drawings. Thedrawings are not necessarily to scale, emphasis instead generally beingplaced upon illustrating the principles of the invention.

FIG. 1 is a diagram of a known mechanized plasma arc torch system.

FIG. 2 is a cross sectional view of a known plasma arc torch tip.

FIG. 3A is an illustration of tolerance measurements used to determinecylindricity of a hole.

FIG. 3B is a cross section of a hole cut with the prior art cuttingprocess.

FIG. 4A is a schematic showing a first zone of a path followed by aplasma arc torch head, according to an illustrative embodiment of theinvention.

FIG. 4B is a schematic showing a second zone of the path followed by aplasma arc torch head, according to an illustrative embodiment of theinvention.

FIG. 4C is a schematic showing a third zone of the path followed by aplasma arc torch head, according to an illustrative embodiment of theinvention.

FIG. 4D is a schematic showing a fourth zone of followed by a plasma arctorch head, according to an illustrative embodiment of the invention.

FIG. 4E shows a method for cutting a hole feature from a workpiece,according to an illustrative embodiment of the invention.

FIG. 4F shows a method for cutting a hole feature from a workpiece,according to another illustrative embodiment of the invention.

FIG. 5A shows a straight lead-in shape for cutting a hole feature,according to an illustrative embodiment of the invention.

FIG. 5B shows a quarter circle lead-in shape for cutting a hole feature,according to an illustrative embodiment of the invention.

FIG. 5C shows a semi-circle lead-in shape for cutting a hole feature,according to an illustrative embodiment of the invention.

FIG. 6A shows a top view of a hole feature where a first zone of a pathfor cutting a workpiece is straight, according to an illustrativeembodiment of the invention.

FIG. 6B shows a bottom view of the hole feature from FIG. 6A, accordingto an illustrative embodiment of the invention.

FIG. 6C shows a top view of a hole feature where a first zone of a pathfor cutting a workpiece is a semi-circle shape, according to anillustrative embodiment of the invention.

FIG. 6D shows a bottom view of the hole feature from FIG. 6C, accordingto an illustrative embodiment of the invention.

FIG. 7 is a graph showing measured deviations for different lead-incommand speeds, according to an illustrative embodiment of theinvention.

FIG. 8 is an exemplary look-up chart for lead-in command speeds,according to an illustrative embodiment of the invention.

FIG. 9 is a schematic of a portion of a hole cut path, according to anillustrative embodiment of the invention.

FIG. 10 is a graph showing a cutting current and a command speed as afunction of time, according to an illustrative embodiment of theinvention.

FIG. 11 is an exemplary look-up chart for cutting parameters, accordingto an illustrative embodiment of the invention.

FIG. 12 is a graph showing measured deviations for a hole feature,according to an illustrative embodiment of the invention.

FIG. 13 shows a method for operating a plasma arc torch to cut a holefeature from a workpiece, according to an illustrative embodiment of theinvention.

FIG. 14 is a graph showing hole quality results for holes cut fromdifferent processes.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 4A-4C show a path 100 for cutting a hole feature (e.g., asubstantially circular hole or rounded slot) from a workpiece, accordingto an illustrative embodiment of the invention. The path can include atleast three zones: a first zone, a second zone, and a third zone. Theterm “zone(s)” as used herein can be defined to include segments orportions of a cut or travel path of a torch head over a workpiece. Insome embodiments, the path includes a fourth zone. The path can definethe motion of the plasma arc torch (e.g., the torch shown in FIG. 1),regardless of whether the cutting current is running or extinguished(i.e., regardless of whether the plasma arc torch is cutting theworkpiece). For purposes of clarity, the individual zones have beendefined in the figures, however, the transitions between zones (e.g.,the transition from the first zone to the second zone, transition fromthe second zone to the third zone, etc.) is not a precise location, butcan be a buffer type region.

FIG. 4A shows the first zone 110 of the path, which can define a“lead-in” of a cut. The plasma arc torch can cut along the first zone110 from a beginning 120 of the first zone to an end 130 of the firstzone. A cut along the first zone of the path 110 can include acorresponding outer kerf edge 140 and a corresponding inner kerf edge150. In this embodiment, the first zone 110 defines a “semi-circle”shaped lead-in cut. The shape of the first zone 110 (e.g., the shape ofthe lead-in) and the command speed are parameters that can affect thequality of the hole feature cut from the workpiece. The command speedcan be a set point for a torch/cutting speed. The torch speed can bedefined as the command speed offset by an acceleration or decelerationof the torch to reach the command speed setpoint and inefficiencies orlimitations inherent in the plasma arc torch system.

FIG. 4B shows a second zone 160 of the path, which can define aperimeter of the cut (e.g., the perimeter of the hole feature). Theplasma arc torch can cut along the second zone 160 from a beginning 170of the second zone 160 to an end 180 of the second zone 160. As shown,the torch head can move from the end 130 of the first zone 110 into thebeginning 170 of the second zone 160. The cut in the second zone 160 caninclude a corresponding outer kerf edge 190 and a corresponding innerkerf edge 200.

FIG. 4C shows the third zone 210 of the path, which can define a kerfbreak-in region (e.g., “lead-out”) of the cut. The plasma arc torch cancut along the path in the third zone 210 from a beginning 220 of thethird zone 210 to an end 230 of the third zone 210 (e.g., where thefirst zone 110, second zone 160, and third zone 210 substantiallyintersect). As shown, the torch head can move from the end 230 of thethird zone 210 into, for example, a location 240 corresponding back tothe beginning 170 of the second zone 160. The cut in the third zone 210can include a corresponding outer kerf edge 250.

The third zone 210 of the path can begin at or near a point 260 wherethe outer kerf edge 140 of the first zone 110 (e.g., the lead-in)substantially intersects with the inner kerf edge 200 of the second zone160. The point 260 as shown in the figure is approximated, as theleading edge 261 of the kerf (e.g., the leading edge of the cut in thesecond zone 160) would break into the outer kerf edge 140 of the firstzone 110 before the inner kerf edge 200. Therefore, the kerf break-inregion would in fact take place where the leading edge 261 of the kerf(e.g., the leading edge of the cut in the second zone 160) would breakinto the outer kerf edge 140 of the first zone 110. However, forpurposes of clarity, the third zone 210 of the path can be defined tobegin at or near point 260. In this embodiment, the third zone 210 ofthe path ends at or substantially near a “0 degree point” 270corresponding to the location where the outer kerf edge 140 of the firstzone 110 (e.g. the lead-in) substantially intersects with the outer kerfedge 250 of the third zone 210 and/or the outer kerf edge 190 of thebeginning 170 of second zone 160. The ramp down of the current in thethird zone 210 and/or varying the command speed of the torch in thefirst zone 110, second zone 160 or third zone 210 are parameters thatcan affect the quality of the feature cut from the workpiece.

In some embodiments, where the hole feature is a circular hole featurewith minimal defects, a distance from a center 280 of the hole featureto an outer kerf edge 190 of the cut in the second zone 160 can besubstantially similar to a distance from the center of the hole featureto an outer kerf edge 250 of the cut in the third zone 210 at a pointwhere the first zone 110 and third zone 210 intersect.

To cut a hole feature from a workpiece, a plasma arc torch can move fromthe first zone 110, to the second zone 160, then to the third zone 210,and then into a fourth zone. The movement of the torch head can follow apath starting from the beginning 120 of the first zone 110, to the end130 of the first zone 110. From the end 130 of the first zone 110, thetorch can move to the beginning 170 of the second zone continuing to theend 180 of the second zone. From the end 180 of the second zone 160, thetorch can move to the beginning 220 of the third zone 210 continuing tothe end 230 of the third zone 210. At the end 230 of the third zone 210,the torch can continue a path 240 that overlaps with the beginning 170of the second zone 160 or to another location on the workpiece. The holefeature can be defined, at least in part, by an outer kerf edge 190 ofthe cut in the second zone 160 and at least a portion of an outer kerfedge 250 of the cut in the third zone 210.

To cut a hole feature in a workpiece, a plasma gas flow can be initiatedand a current flow can be initiated to ignite a pilot arc. The arc canbe transferred to the workpiece. In some embodiments, a plasma arc torchbegins cutting a hole feature in a workpiece by piercing the workpieceat a first location (e.g., a beginning 120 of the first zone 110 orpoint 280) and cuts a semicircle in the workpiece (e.g., a semi-circlepath) along the first zone 110 of the path. The plasma arc torch can cutalong the first zone 110 and “lead-in” to the second zone 160 of thepath and begin cutting the perimeter of the hole feature. The plasma arctorch can cut along the second zone 160 of the path into the third zone210 of the path. In some embodiments, the plasma arc torch cuts alongthe second zone 160 of the path using O₂ plasma gas and O₂ shield gas.The plasma arc torch can cut the workpiece in the third zone 210 whileeither substantially maintaining the command speed of the torch or whileincreasing the command speed of the torch. The plasma arc torch cancontinue to cut the workpiece in the third zone 210 of the path until itreaches at or substantially near a “0 degree point” 270 (e.g., where theouter kerf edge 140 of the first zone 110 (e.g. the lead-in)substantially intersects with the outer kerf edge 250 of the third zone210 and/or the outer kerf edge 190 of the beginning 170 of second zone160). The cutting current can be ramped down in the third zone 210 suchthat the cutting current is extinguished and/or the arc is “shut off”(e.g., the plasma arc torch stops cutting the workpiece) as the plasmaarc torch reaches at or near this “0 degree point” 270 (e.g., the arcshut off point). The torch head can continue moving past the “0 degreepoint” 270 even though the torch is no longer cutting the workpiece.After the torch head reaches the “0 degree point” 270 and after the archas been extinguished, the torch head can be decelerated along a path240. The torch head can be decelerated while moving a path 240 thatoverlaps with the second zone 160. If the torch is decelerated along apath 240 that follows the circular path of the hole feature, as shown inFIG. 4C, the following equations can be used to calculate the “lead-out”motion angle (e.g., the angle traveled by the torch head after the “0degree point” 270 after which the torch is decelerated to a stop) andthe minimum “lead-out” motion length (e.g., the distance traveled by thetorch head after the “0 degree point” 270 after which the torch isdecelerated to a stop):L=V ²/(2·a)  EQN. 1Φ=360·L/(Π·D)  EQN. 2where “L” is the minimum lead-out motion length, “V” is the velocity ofthe torch head, “a” is a table deceleration and “D” is the motiondiameter of the hole feature. In some embodiments, the torch head beginsto decelerate only after the current has been extinguished. The minimumlead-out motion length “L” can be defined as the minimum distancerequired between the “0 degree point” 270 and the point where the torchdecelerates to a stop to ensure that the torch does not begindecelerating prior to the “0 degree point” 270. The torch head can becommanded to decelerate to a stop at point 290 a predetermined distanceafter the “0 degree point” 270. In some embodiments, the minimumlead-out motion length is about ¼ of an inch.

FIG. 4D shows a fourth zone 240 or 240′ of a path (e.g., a “decelerationzone”) for a plasma arc torch head, according to an illustrativeembodiment of the invention. As noted above, the plasma arc torch canstop cutting (e.g., the cutting current extinguished) as the plasma arctorch reaches at or substantially near the “0 degree point” 270 (e.g.,where the outer kerf edge 140 of the first zone 110 substantiallyintersects with the outer kerf edge 250 of the third zone 210 and/or theouter kerf edge 190 of the beginning 170 of second zone 160). The plasmaarc torch head can enter a fourth zone 240 or 240′ of the path anddecelerate in the fourth zone.

In some embodiments, the fourth zone 240 substantially overlaps in spacewith a beginning 170 the second zone 160. In some embodiments, the torchhead can travel in the fourth zone 240′ where the path extends toanother location on the workpiece. In some embodiments, the torch headcan be decelerated to a stop at point 290 or 301 located at apredetermined distance (e.g., ¼ of an inch) after the “0 degree point”270 so that the torch does not begin to decelerate until the arc issubstantially extinguished.

A method for cutting a hole feature from a workpiece can include cuttingin the first zone 110 using at least one cutting parameter from a firstcutting parameter set, cutting in the second zone 160 using at least onecutting parameter from a second cutting parameter set, and cutting inthe third zone 210 using at least one cutting parameter from a thirdcutting parameter set. The first cutting parameter set can include afirst cutting current or a first command speed establishing a firsttorch speed. The command speed can be a setpoint for the speed set by auser, CNC, or computer program, etc. The torch/cutting speed can bedefined as the command speed offset by an acceleration/deceleration ofthe torch to reach the command speed setpoint and anyinefficiencies/limitations inherent in the plasma arc torch system. Thesecond cutting parameter set can be different from (e.g., have at leastone parameter different from) the first cutting parameter set and caninclude a second cutting current or a second command speed (e.g.,greater than the first command speed) establishing a second torch speed.The third cutting parameter set can be different from (e.g., have atleast one parameter different from) the first cutting parameter set orthe second cutting parameter set and can include a third cutting current(e.g., less than the second cutting current) or a third command speedestablishing a third torch speed. The first, second and third parametersets can be independent from one another (e.g., the parameters areselected independently of one another).

In some embodiments, no two parameter sets are identical. For example,the plasma arc torch system can change a command speed whentransitioning from the first zone to the second zone (e.g., increase thecommand speed so that the command speed in the second zone is higher). Ahigher command speed (e.g., resulting in a higher torch speed) can beused to cut the second zone (e.g., perimeter) than the first zone (e.g.,lead in) so as to minimize changes in centripetal acceleration andminimize dynamic responses by the arc. If a command speed were to besubstantially maintained between the first zone and the second zone, thecentripetal acceleration in the first zone would be greater than thesecond zone, which can result in dynamic responses by the cutting arcand unwanted defects. The plasma arc torch system can also change acutting current when transitioning from the second zone to the thirdzone (e.g., ramp down the cutting current in the third zone). The plasmaarc torch system can also change the torch speed when transitioning fromthe third zone to the fourth zone (e.g., begin to decelerate the torchafter the cutting current has been extinguished).

FIG. 4E shows a method for cutting a hole feature from a workpiece,according to an illustrative embodiment of the invention. The method caninclude step 310 of cutting in the first zone 110 using a first commandspeed establishing a first torch speed, step 320. The method can alsoinclude cutting in the second zone 160 using a second, different,command speed establishing a second torch speed, such that the torchspeed is increased when moving from the first zone to the second zone.In some embodiments, the first command speed and the second commandspeed are part of an acceleration curve (e.g., where the second commandspeed is greater than the first command speed). In some embodiments, theworkpiece is cut at a first torch speed (e.g., established by a commandspeed) where the first zone and the second zone substantially intersectand a second, greater, torch speed is used to cut at least a portion ofthe second zone (e.g., a majority of the second zone). As noted above,the torch speed can be increased when moving from the first zone to thesecond zone so as to minimize changes in centripetal acceleration anddynamic responses by the cutting arc which can result in unwanteddefects. The method can also include step 330 of ramping down a cuttingcurrent (e.g., reducing a cutting current) in the third zone 210 suchthat the cutting current reaches substantially zero amperes (step 350)at a location/point corresponding to a beginning 170 of the second zone210 where the first zone 110, second zone 160 and third zone 210substantially intersect (e.g., at or substantially near the “0 degreepoint” 270 as shown in FIGS. 4C and 4D). The method can includesubstantially maintaining or further increasing the command speed in thethird zone during ramp down of the current (step 340). The method caninclude decelerating a torch speed of the plasma cutting system afterthe cutting current has reached substantially zero (Step 360).

The command speed of the cut for the second zone 160 can be different(e.g., greater than) than a command speed of the cut in the first zone110. The first command speed can be based at least in part on a diameterof the hole feature. The third zone 210 can be cut with a command speed(e.g., a third command speed) greater than the command speed of thesecond zone 160 of the cut. The torch speed can be decelerated after apoint where the outer kerf edge 140 of the cut in the first zone 110substantially intersects with the outer kerf edge 250 of the cut in thethird zone 210 (e.g., the “0 degree point” 270 as shown in FIGS. 4C and4D). The torch speed (e.g., the actual speed of the torch head) can bedecelerated to reach zero at a predetermined distance after the pointwhere the outer kerf edge 140 of the cut in the first zone 110substantially intersects with the outer kerf edge 250 of the cut in thethird zone 210.

The plasma cutting current in the third zone 210 (e.g., the thirdcutting current) can be extinguished when a torch head passes from thethird zone 210 to the second zone 160 at a location where the first zone110, second zone 160 and third zone 210 substantially intersect. Thecutting current can be reduced (e.g., ramped down) in the third zone 210such that the cutting current reaches substantially zero amperes at apoint/location where an outer kerf edge 250 of the cut in the third zone210 substantially meets an outer kerf edge 140 of the cut in the firstzone 110. The ramp down for the cutting current in the third zone 210can be ramped down at a rate based, at least in part, upon a lengthbetween a beginning 220 of the third zone 210 and the beginning 170 ofthe second zone 160. Alternatively, the rate at which the cuttingcurrent is ramped down can be a function of a diameter of the holefeature to be cut from the workpiece. A ramp down of the cutting currentcan be initiated at a location in the third zone 210 determined by thethird torch speed and a ramp down time (e.g., the time required for thecurrent to reach substantially zero amperes).

The plasma arc torch can cut in the first zone 110, second zone 160,and/or the third zone 210 using a gas flow composition of O2 plasma gasand O2 shield gas (e.g., or low N₂ gas composition) to reduce defectssuch as bevel and/or taper of the hole feature.

A plasma arc torch system (e.g., as shown in FIG. 1) can be used to cuta hole feature in a workpiece along a first zone 110, a second zone 160,and a third zone 210. The plasma arc torch system can include a plasmatorch 24 including an electrode 27 and a nozzle 28, a lead that providesa cutting current to the plasma arc torch 24, a gantry 26 that moves theplasma torch and a CNC 12 that controls cutting parameters of the plasmaarc torch in the first zone 110, the second zone 160, and the third zone210. A CNC 12 can select cutting parameters for cutting a hole featurein a workpiece. A computer readable product, tangibly embodied on aninformation carrier, and operable on a CNC 12, can include instructionsbeing operable to cause the CNC 12 to select the cutting parameters. TheCNC 12 can establish a first command speed for the first zone 110 and asecond command speed for the second zone 160. The first command speedcan be based, at least part, on a diameter of the hole feature. Thesecond command speed can be greater than the first command speed. TheCNC 12 can also establish a third cutting current for the third zone 210and ramp down the third cutting current so that the third cuttingcurrent reaches substantially zero amperes where an outer kerf edge 140of the cut in the first zone 110 substantially intersects with an outerkerf edge 250 of the cut in the third zone 210. The CNC 12 can include alook-up table to identify the cutting parameters of the plasma arctorch.

FIG. 4F shows a method for cutting a hole feature from a workpiece,according to another illustrative embodiment of the invention. To cut ahole feature from a workpiece, the plasma gas flow can be initiated, acurrent flow can be initiated to ignite a pilot arc and the arc can betransferred to the workpiece. The workpiece can be pierced (e.g., tobegin cutting an internal feature/hole feature in the workpiece). Acutting arc and a cut speed can be established with respect to theworkpiece (e.g., piercing the workpiece at point 370 and setting acommand speed that defines the cut speed). The cut speed can beincreased to a second cut speed after a first point 380 (e.g., after theend of the first zone as described above in FIG. 4A) in a hole cut path.In some embodiments, the plasma arc torch can cut the workpiece in thethird zone so that the amount of current per linear distance traveledreduces as the torch cuts along the third zone. A cutting current can beramped down after a second point 390 in the hole cut path (e.g., afterthe end of the second zone and at or after the beginning of the thirdzone as described above in FIGS. 4B-4C) either while substantiallymaintaining the cut speed or while increasing the cut speed. The plasmaarc torch can also substantially maintain the cutting current butincrease the command speed (e.g., thereby increasing the torch speed)while cutting in the third zone. The torch can cut from the second point390 in the hole cut path and return back to the first point 380 in thehole cut path to form the hole feature in the workpiece. The cuttingcurrent can be ramped down while cutting from the second point 390 inthe hole cut path back to the first point 380 in the hole cut path. Thecutting arc can be extinguished substantially near the first point 380in the hole cut path (e.g., near the “0 degree point” 270 shown in FIGS.4C-4D). The second cut speed can either be maintained until the cuttingcurrent reaches substantially zero amperes or the second cut speed canbe increased to a third cut speed before the cutting current reachessubstantially zero amperes (e.g., at or near first point 380).

FIGS. 5A-5C show different shapes for the first zone of the path (e.g.,lead-in shapes) that can be used in cutting a hole feature from aworkpiece, according to illustrative embodiments of the invention.Different lead-in shapes can be used to cut a hole feature from aworkpiece. FIG. 5A shows an exemplary straight lead-in shape 110A forcutting a hole feature. FIG. 5B shows an exemplary quarter circle 110Blead-in shape for cutting a hole feature. FIG. 5C shows an exemplarysemi-circle lead-in shape 110C for cutting a hole feature.

FIGS. 6A-6D show the results of hole features cut from a workpiece usinga semi-circle shaped path in the first zone (e.g., semi circle lead-in)and a hole feature cut from a straight path in the first zone (e.g., astraight lead-in). FIGS. 6A-6D show “transition” points 400A-D where theend 230 of the third zone 210 substantially intersects the beginning 170of the second zone 160 (e.g., where lead-in of the cut meets theperimeter of the cut and the kerf break-in region (e.g., “lead-out”) ofthe cut). FIG. 6A shows a top view of a hole feature cut from aworkpiece using a straight shape for the first zone of the path (e.g.,110A of FIG. 5A). FIG. 6B shows a bottom view of the hole feature ofFIG. 6A. FIG. 6C shows a top view of a hole feature using a semi-circlelead-in shape (e.g., 110C of FIG. 5C). FIG. 6D shows a bottom view ofthe hole feature cut from a workpiece of FIG. 6C. As shown in FIGS.6A-6B, a hole feature cut using a straight-shaped first zone (e.g., astraight lead-in or 110A of FIG. 6A) resulted in unwanted defects, suchas a protrusion. As shown in FIGS. 6C-6D, a hole feature cut using afirst zone shaped like a semi-circle 110C (e.g., a semi-circle lead-in)generated a hole feature with less defects than the hole feature cutusing a straight lead-in.

FIG. 7 is a graph showing measured deviations for different lead-incommand speeds, according to illustrative embodiments of the invention.The graph shows measurements of deviations in the hole feature in thetransition area where the first zone, second zone and third zone asshown in FIGS. 4A-4C merge. The graph shows deviations (“D leveldeviations”) for different “lead-in speeds” (e.g., command speed set forcutting along the first zone as shown in FIG. 4A). Specifically, thegraph shows “D level deviations” which are deviations measured in thehole feature +90 degrees counterclockwise from the “0 degree point” 270and −90 degrees clockwise from the “0 degree point” 270 as shown inFIGS. 4C-4D. The “0.000” line is where the hole should be if the holewere a “perfect” hole free of defects/deviations. Points above and belowthe “0.000 line” indicate deviations/defects such as protrusions anddivots, respectively. There are smaller deviations/defects at the −90degree point and +90 degree point and greater deviations/defects nearthe “0 degree point” (e.g., point 270 in FIGS. 4C-4D). As compared tocylindricity (as described above for FIG. 3A), which encompasses thethickness and the perimeter of the hole feature, the deviations in FIG.7 reflect a segment/portion of the hole feature at a given depth. Whileit can be desirable for the lead-in speed (e.g., command speed for thefirst zone of the path) to be less than the speed for cutting theperimeter of the hole (e.g., the command speed for the second zone ofthe path), there exists an optimal speed for the hole feature.Optimizing the lead-in speed can further reduce deviations (e.g., divotsor protrusions) at positive angles (e.g., in the second zone where thefirst zone transitions into the beginning of the second zone).

A method to measure deviations in the hole feature can include the stepof scanning, at a depth near the bottom of the hole, a half circle ofthe hole feature away from the lead-in and arc shut off region (e.g.,scanning a portion of the second zone/perimeter of the hole, forexample, clockwise from the −90 degree point to the +90 degree point orcounterclockwise from the +90 degree point to the −90 degree point asshown in FIGS. 4A-4D) to determine the hole diameter and centerlocation. The method can include a second step of scanning, at about thesame depth, the lead-in and arc shut off region of the hole (e.g.,scanning the third zone and the beginning of the second zone, forexample, clockwise from the +90 degree point to the −90 degree point orcounterclockwise from the −90 degree point to the +90 degree point asshown in FIGS. 4A-4D) and calculate deviations from the measured holediameter and location (e.g., by comparing the measurements obtained byscanning away from the lead-in and arc shut off region with themeasurements obtained by scanning the lead-in and arc shut off region).As noted above the current can be extinguished at or substantially nearthe “0 degree point” 270 (e.g., where the outer kerf edge of the firstzone intersects with the outer kerf edge of the third zone as shown inFIG. 4C-4D), thus defining the “arc shut off region.” The deviation datacan be plotted at each angular position. To determine the optimalprocess for cutting a hole feature, the lead-in speed having minimaldefects (e.g., deviation values closest to zero) in the region can beselected for each hole size.

FIG. 7 is a deviation plot for various lead-in speeds (e.g., variousspeeds for the first zone of the path as shown in FIG. 4A). The holeswere 0.394 inch diameter holes cut from ⅜″ Mild Steel using a cuttingcurrent of 130 amperes and a gas composition of O2/O2 (plasma gas/shieldgas). A cutting speed for the perimeter (e.g., second zone in FIG. 4B)of the feature was set at about 45 ipm. The optimal lead-in speed forthis process, material and hole size were speeds set at about 25-27 ipm(plots 440 & 450). For example, hole features cut from a lead-in speedset at about 40 ipm (plot 480) produced greater defects (i.e.,protrusions measured at about 0.023 inches) than holes cut from a cutlead-in speed set at about 25-27 ipm. Hole features cut from a lead-inspeed set at about 20 ipm (plot 410) produced greater defects (i.e.,divots measured at about −0.013 inches) than holes cut from a cutlead-in speed set at about 25-27 ipm. In contrast, hole features cut atlead-in speeds of about 25-27 ipm produced protrusions (e.g., in portionof the second zone defined clockwise from the +90 degree to the 0 degreepoint) measured at about 0 inches to about 0.002 inches.

Typically, the optimal lead-in speeds are reduced as the hole diametergets smaller. A plot of optimal lead-in speed as a function of holediameter could be tested, similar to the plot shown in FIG. 7, developedand curve fit to an equation. The coefficients for the equation couldappear in the hole cut chart which could be read and used in thecalculations performed by the CNC. FIG. 8 is an exemplary look-up chart570 for lead-in command speeds, according to an illustrative embodimentof the invention. As shown in FIG. 8, the optimal lead-in speed can be afunction of hole diameter and can vary based on the cutting currentlevel and thickness of the workpiece. The size of the hole feature canbe directly related to the magnitude of the lead-in speed. For example,smaller hole features can be cut using lower lead-in speeds and largerhole features can be cut using greater lead-in speeds. For example, aprocess for cutting a 0.276 inch diameter hole feature from a 0.375 inchmild steel workpiece at 130 Amps can have an optimal lead-in speed setat about 12 ipm to minimize defects in the hole feature. In contrast, aprocess for cutting a 0.315 inch diameter hole feature from a 0.375 inchmild steel workpiece at 130 Amps can have an optimal lead-in speed setat about 19 ipm to minimize defects in the hole feature.

FIG. 9 shows a third zone for a path used in cutting a hole feature froma workpiece, according to an illustrative embodiment of the invention.The motion of the torch head can follow path 210. The third zone canextend from a point where the outer kerf edge 140 of the first zone 110(e.g., the lead-in of the cut) intersects with an inner kerf edge 200 ofthe second zone 160 to the “0 degree point” 270 (e.g., where the outerkerf edge 140 of the first zone 110 merges with the outer kerf edge 250of the third zone 210). As noted above, this is an approximation as theleading edge 261 of the cut will intersect the outer kerf edge of thefirst zone before the inner kerf edge 200 of the second zone 160intersects. When the torch angular position (Φ) 580 while cutting thehole reaches Φref, the kerf leading edge breaks into the lead-in (e.g.,first zone) outer kerf edge 140, approximately where third zone 210begins. As Φ 580 decreases, the amount of material remaining decreasesreaching zero at Φ=0 (“0 degree point”). The remaining material (e.g.,the “diminishing material” 590) can be calculated as a function of Φ.

The diminishing material 590 can be the leftover material from theworkpiece to be cut once the torch head reaches the beginning 220 of thethird zone 210. The diminishing material 590 can be defined at least inpart by an outer kerf edge 140 of the cut in the first zone 110 and anouter kerf edge 250 of the cut in the third zone 210. Removing too muchmaterial can cause divots in the hole feature, while not removing enoughmaterial can cause protrusions in the hole feature. Therefore, it isdesirable to optimize the material removed in the third zone 210 so thatan outer kerf edge 250 of the cut in the third zone 210 substantiallyaligns with an outer kerf edge 180 of the cut in the second zone 160. Asthe amount of remaining material to be removed (e.g., the diminishingmaterial 590) to cut the hole feature varies along the third zone 210(e.g., from the beginning 220 of the third zone 210 to the end 230 ofthe third zone 210 where the first zone 110, second zone 160, and thirdzone 210 substantially intersect), the current density used to cut theworkpiece as the torch travels along the third zone 210 can be optimizedso that the correct amount of material is removed from the workpiece. Aramp down of the cutting current (e.g., the third cutting current)and/or varying the torch speed (e.g., the cutting speed) can beoptimized to provide the desired amount of current density per lineardistance of the travel by the torch in the third zone 210. A cuttingcurrent can be ramped in the third zone 210 to remove a diminishingmaterial 590 such that an outer kerf edge 250 of the cut in the thirdzone 210 substantially aligns with an outer kerf edge 180 of the cut inthe second zone 160. The method can also include substantiallymaintaining or increasing a torch speed in the third zone 210 until thetorch head passes from the third zone 210 into a location correspondingto the second zone (e.g., zone 240 shown in FIG. 4C).

FIG. 10 is a graph 600 showing a cutting current and a command speed asa function of time, according to an illustrative embodiment of theinvention. The process current 610 can be signaled 620 (e.g., by a CNC)to ramp down and extinguish at or substantially near the “0 degreepoint” 270′ (e.g., where the outer kerf edge 140 of the first zone 110merges with the outer kerf edge 250 of the third zone 210 as shown inFIG. 4C and FIG. 9). There can be a propagation delay 630 between whenthe signal 620 to ramp down the current is sent and when the currentlevel actually begins to ramp down. The torch speed 640 (e.g., the torchspeed) can be decelerated after the torch head passes the “0 degreepoint” 270′ and after the current has been extinguished (e.g., after thecutting current reaches substantially zero amperes).

At least one of a plurality of cutting current ramp down operations forcutting in the third zone can be selected, where each of the pluralityof cutting current ramp down operations can be a function of a diameterof the hole feature. The cutting current can be ramped down at a firstpoint in the third zone 210′ (e.g., at or near the beginning of thethird zone) such that the cutting current 610 is extinguished at orsubstantially near a second point 270′ (e.g. corresponding to point 270as shown in FIGS. 4C-4D) in the third zone 210 where the first zone, thesecond zone and the third zone 210′ substantially intersect (e.g., nearthe end 230′ of the third zone 210′). The first point in the third zonecan be determined/calculated using a ramp down time of the cuttingcurrent. The plasma arc torch can be decelerated so that a torch speedreaches substantially zero at a predetermined distance 650 (e.g., ¼″)after the second point 270′.

The process current 610 ramp down and shut off in the third zone 210′can be performed at full process cutting speed (e.g., by substantiallymaintaining the command speed) or at a higher torch speed than thesecond zone (e.g., by increasing the command speed). The current shutoff 610 can be substantially coincident with the 0 degree mark 270(e.g., the lead-in/hole transition location or location where the firstzone transitions into the second zone). In some embodiments, the torchhead can be decelerated 640 to a stop at a predetermined distance 650(e.g., ¼ of an inch) after the “0 degree mark” 270′ equal to or greaterthan the minimum “lead-out” motion length (as described above). Theplasma can be signaled 620 to ramp down at a point in time correspondingto the current ramp down time and propagation delay so that the currentis extinguished when the torch reaches at or near the “0 degree point”270. Therefore, the time interval between the point where the plasmacurrent is signaled 620 to ramp down and the point 270 where the currentis extinguished can correspond to the ramp down time.

FIG. 11 is an exemplary look-up chart 660 for cutting parameters,according to an illustrative embodiment of the invention. The ramp downtime of the current can be included in the cut chart 660. The currentramp down time can vary depending on the current level of the process.By way of example, a process operating with a current level of 400 A cantake about 250 ms to ramp down (e.g., to extinguish the current). Aprocess operating with a current level of about 50 Amps can take about50 ms to ramp down. Therefore, to extinguish the current at the “0degree point” (e.g., where the outer kerf edge of the first zone mergeswith the outer kerf edge of the third zone as shown in FIG. 4C and FIG.9), the CNC can signal the plasma arc torch system to ramp down thecurrent to extinguish the current at a point based, at least, on acommand speed and the ramp down time (e.g., the time taken for thecurrent to be extinguished).

A minimum deceleration time can be defined as the minimum time that canbe set for a plasma arc torch to decelerate to a stop, so that theplasma arc torch does not begin to decelerate until the arc isextinguished and the torch reaches the “0 degree point” as describedabove in FIGS. 4C-4D. The minimum deceleration time can be calculated byusing an upper limit for the torch head speed (e.g., an 80 Amp processon a ¼″ Mild Steel workpiece can use a cut speed of about 55 ipm) and alower limit for the table deceleration (e.g., a “slow table” can have atable deceleration of about 5 mG). EQN. 1 above can be used to calculatethe minimum lead-out motion length “L” (e.g., the minimum distance thatcan be set so that the table does not begin decelerating until after thecurrent has been extinguished and/or after the torch head hassubstantially reached the “0 degree point,” the minimum distancecalculated for a fast torch speed and slow table) which can be about0.25 inches given a torch speed of about 55 ipm and a table decelerationof about 5 mG. Therefore, for most processes, a plasma arc torch can becommanded to decelerate to a stop 0.25 inches after the “0 degree point”so that the plasma arc torch maintains the torch head speed until thecurrent is extinguished. A process with a lower speed (e.g., a slowertorch speed) and/or or a faster table (e.g., a greater tabledeceleration) will come to a stop before it reaches 0.25 inches afterthe “0 degree point” but the table will still decelerate after thecurrent has been extinguished. In an alternative embodiment, the CNCnegative cut off time to decelerate the torch head (e.g., to a stop) canbe set to the sum of the propagation delay 630, process ramp down 610(e.g., the time taken to ramp down the current), any additional lead-outand motion deceleration times 650.

As described above in FIG. 7, FIG. 12 shows measurements of deviationsin the hole feature in the transition area where the first zone, secondzone and third zone as shown in FIGS. 4A-4C merge. Specifically, thegraphs show deviations in the hole feature +90 degrees counterclockwisefrom the “0 degree point” and −90 degrees clockwise from the “0 degreepoint” as shown in FIGS. 4A-4D. Graph 670 shows measured deviations fora hole feature, according to an illustrative embodiment of theinvention. Plot 661 shows the outer kerf edge of the first zone, asshown in FIG. 4A. To minimize the depth of the lead-in/lead-out formerror “ding” and/or “divot”, the process ramp down (e.g., ramping downon the current in the third zone as shown in FIGS. 4C and 10) can beperformed by substantially maintaining or increasing the full processcutting speed (e.g., increasing the cutting speed by setting a highercommand speed). The average deviation (e.g., divot) produced in thethird zone (e.g., between −90 degrees and 0 degrees as shown in FIG. 4C)for the holes was about −0.020 inches.

FIG. 13 shows a method for operating a plasma arc torch to cut a holefeature from a workpiece, according to an illustrative embodiment of theinvention. Software can generate code for the CNC to execute andinstruct the plasma system to perform a number of steps during operationof the torch. A step can include locking out height control for theplasma arc torch (step 800). The gas composition (e.g., O2/O2 forplasma/shield gas) can be set from the lookup chart (step 810). The kerfvalue (e.g., from a lookup chart) can be used to calculate the motiondiameter from the hole feature diameter to be cut (step 820). A step caninclude reading/setting the command speed from values in the hole cutchart (step 830). The path used to cut the hole feature can beprogrammed to include a 360 degree arc, an asynchronous stop command,and a lead-out arc length (e.g., an arc in the fourth zone as shown inFIG. 4D) (step 850). The asynchronous stop command can be a command thattells the plasma arc torch to extinguish the plasma arc, but to continuemoving the torch head. The torch stop command (e.g., the command todecelerate the torch to a stop) can then be given later. Theasynchronous stop command can be inserted at the “0 degree point” (e.g.,point 270 as shown in FIGS. 4C-4D) and can include an offset with a timeinterval that corresponds to the ramp down time so that the current isextinguished to substantially zero at the “0 degree point” (Step 840).The lead-out arc length can be programmed to correspond to the minimumlead-out motion length (e.g., as described above in EQN. 1), so that thetorch does not begin to decelerate until after the “0 degree point”(e.g., so that the torch speed is not lowered until the current isextinguished). A lead-in shape (e.g., a semi-circle) from the holecenter to the motion diameter (step 860) can be programmed. A lead-inspeed can be read/set from values in the hole cut chart (e.g., commandspeeds for the first zone as described above) (step 870). The currentramp down time can be read from a look up table for any number of holessizes and any significant propagation delay can be added to the timeinterval that offsets the asynchronous stop command as described above.Thus, an appropriate current ramp down time can be calculated for anynumber of hole sizes.

FIG. 14 is a graph comparing hole quality results for holes cut fromdifferent processes. The graph shows the cylindricity for hole featurescut using different cutting processes 880-920. Cylindricity can be afunction of hole size (e.g., hole diameter). The hole features cut usingprocesses 880-920 in FIG. 14 were each 0.394 inches in diameter, andwere cut in ⅜″ thick mild steel workpieces. Processes 900-910 are holefeatures cut from processes incorporating exemplary features of theembodiments of the invention described herein. Process 920 was a holefeature cut using a laser cutting system. While laser cutting systemshave previously yielded higher quality holes (e.g., comparing Process920 to, for example, Process 880), plasma arc torch systems are lower incost. Therefore, there is a need for high quality holes cut from plasmaarc torch systems.

The plot for Process 880 (“Benchmark Plasma”) shows the cylindricity fora hole feature cut from existing methods. The gas composition forProcess 880 used O₂ plasma gas and Air for shield gas. A straightlead-in (e.g., straight cut for the first zone) was used and the torchwas decelerated prior to the “0 degree point” (e.g., prior toextinguishing the current). The cylindricity for the hole feature cutfrom Process 880 was about 0.059 inches.

The plot for Process 890 (“Partial A”) shows the cylindricity for a holefeature cut where the only change from Process 880 was the gascomposition. The gas composition for Process 890 was O₂ plasma gas andO₂ shield gas. The cylindricity for the hole feature cut from Process890 was about 0.100 inches. Therefore, merely changing the shield gascomposition and flow rate from Process 880 amplified the defects in thehole feature.

The plot for Process 900 (“Partial B”) shows the cylindricity for a holefeature where a semi-circle shape was used to cut the first zone (e.g.,a semicircle lead in) and where the command speeds for the second zonewere higher than the command speed for the first zone. The torch,however, was decelerated before the current was extinguished (e.g.,before the “0 degree point” as shown in FIGS. 4C-4D). The gascomposition for Process 900 was O₂ plasma gas and O₂ shield gas. Thecylindricity for the hole feature cut from Process 900 was about 0.039inches. Therefore, this data shows that changing the command speedsalong the cut and choosing a semi-circle lead-in improved the holequality. A hole cut feature cut by this process has a lower cylindricitythan a hole cut by using a process that has a straight lead-in and notorch speed change between the first and second zones.

The plot for Process 910 (“Full Solution”) shows the cylindricity for ahole feature where a semi-circle shape was used to cut the first zone,where the command speeds for the second zone were higher than thecommand speed for the first zone and where the torch was deceleratedafter the current was extinguished (e.g., after the “0 degree point” asdescribed above). The gas composition for Process 910 was O₂ plasma gasand O₂ shield gas. The cylindricity for the hole feature cut fromProcess 910 was about 0.020 inches, thereby showing improvement in cutquality as compared to the other plasma arc torch processes. A holefeature cut by this process has a lower cylindricity than a hole featurecut by a torch that was decelerated before the current was extinguished.

The plot for Process 920 (“Laser”) shows the cylindricity for a holefeature cut using a laser cutting system. The cylindricity for the holefeature cut from a laser system was about 0.015 inches. Cutting a holefeature incorporating the aspects/features of the embodiments describedherein, as shown in plots for Processes 900-910, improved the quality ofholes cut using plasma arc torch systems.

The above-described techniques can be implemented in digital electroniccircuitry, or in computer hardware, firmware, software, or incombinations of them. The implementation can be as a computer programproduct, i.e., a computer program tangibly embodied in an informationcarrier (e.g., a CPS). An information carrier can be a machine-readablestorage device or in a propagated signal, for execution by, or tocontrol the operation of, data processing apparatus (e.g., aprogrammable processor, a computer, or multiple computers).

A computer program (e.g., a computer program system) can be written inany form of programming language, including compiled or interpretedlanguages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A computer program can bedeployed to be executed on one computer or on multiple computers at onesite or distributed across multiple sites and interconnected by acommunication network.

Method steps can be performed by one or more programmable processorsexecuting a computer program to perform functions of the invention byoperating on input data and generating output. Method steps can also beperformed by, and apparatus can be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit). Modules can refer to portionsof the computer program and/or the processor/special circuitry thatimplements that functionality.

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. The essential elements of a computer area processor for executing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, (e.g.,magnetic, magneto-optical disks, or optical disks). Data transmissionand instructions can also occur over a communications network.Information carriers suitable for embodying computer programinstructions and data include all forms of non-volatile memory,including by way of example semiconductor memory devices, e.g., EPROM,EEPROM, and flash memory devices; magnetic disks, e.g., internal harddisks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROMdisks. The processor and the memory can be supplemented by, orincorporated in special purpose logic circuitry.

To provide for interaction with a user, the above described techniquescan be implemented on a computer having a display device, e.g., a CRT(cathode ray tube) or LCD (liquid crystal display) monitor, fordisplaying information to the user and a keyboard and a pointing device,e.g., a mouse or a trackball, by which the user can provide input to thecomputer (e.g., interact with a user interface element). Other kinds ofdevices can be used to provide for interaction with a user as well; forexample, feedback provided to the user can be any form of sensoryfeedback, e.g., visual feedback, auditory feedback, or tactile feedback;and input from the user can be received in any form, including acoustic,speech, or tactile input.

The above described techniques can be implemented in a distributedcomputing system that includes a back-end component, e.g., as a dataserver, and/or a middleware component, e.g., an application server,and/or a front-end component, e.g., a client computer having a graphicaluser interface and/or a Web browser through which a user can interactwith an example implementation, or any combination of such back-end,middleware, or front-end components. The components of the system can beinterconnected by any form or medium of digital data communication,e.g., a communication network. Examples of communication networksinclude a local area network (“LAN”) and a wide area network (“WAN”),e.g., the Internet, and include both wired and wireless networks.

Comprise, include, and/or plural forms of each are open ended andinclude the listed parts and can include additional parts that are notlisted. And/or is open ended and includes one or more of the listedparts and combinations of the listed parts.

While the invention has been particularly shown and described withreference to specific illustrative embodiments, it should be understoodthat various changes in form and detail may be made without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A method for cutting a hole feature in aworkpiece along at least a portion of a path including a first zone, asecond zone, and a third zone using a plasma cutting system, the methodcomprising: cutting in the first zone using at least one cuttingparameter from a first cutting parameter set, the first cuttingparameter set comprising a first cutting current and a first commandspeed establishing a first torch speed; cutting in the second zone usingat least one cutting parameter from a second cutting parameter setdifferent from the first cutting parameter set, the second cuttingparameter set comprising a second cutting current and a second commandspeed establishing a second torch speed; and cutting in the third zoneusing at least one cutting parameter from a third cutting parameter setdifferent from the first cutting parameter set or the second cuttingparameter set, the third cutting parameter set comprising a thirdcutting current and a third command speed establishing a third torchspeed.
 2. The method of claim 1 wherein the first zone corresponds to alead-in of a cut, the second zone corresponds to a perimeter of the cut,and the third zone corresponds to a kerf break-in region of the cut. 3.The method of claim 2 wherein the hole feature is defined at least inpart by an outer kerf edge of the cut in the second zone and at least aportion of an outer kerf edge of the cut in the third zone.
 4. Themethod of claim 1 wherein cutting in the first zone comprises cutting atleast a semi-circle in the workpiece.
 5. The method of claim 1 whereinthe second command speed is greater than the first command speed.
 6. Themethod of claim 1 wherein the third cutting current is less than thesecond cutting current during at least a portion of the third zone.
 7. Amethod for cutting a hole feature in a workpiece along at least aportion of a path including a first zone and a second zone using aplasma cutting system, the method comprising: initiating a plasma gasflow; initiating a current flow to ignite a pilot arc; transferring thearc to the workpiece; piercing the workpiece; cutting in the first zoneusing a first command speed establishing a first torch speed; andcutting in the second zone using a second command speed establishing asecond torch speed, wherein the second command speed is greater than thefirst command speed.
 8. The method of claim 7 wherein the path includesa third zone and wherein the first zone corresponds to a lead-in of acut, the second zone corresponds to a perimeter of the cut, and thethird zone corresponds to a kerf break-in region of the cut.
 9. Themethod of claim 7 wherein the first command speed is based at least inpart on a diameter of the hole feature.
 10. The method of claim 7wherein the hole feature is a substantially circular hole or slot. 11.The method of claim 7 wherein the path includes a third zone, the methodfurther comprising ramping down a cutting current in the third zone suchthat the cutting current reaching substantially zero amperes at alocation corresponding to a beginning of the second zone where the firstzone, second zone and third zone substantially intersect.
 12. The methodof claim 11 further comprising reducing the torch speed after thecutting current reaches substantially zero amperes.
 13. The method ofclaim 11 further comprising ramping down the cutting current at a ratebased at least in part upon a length between a beginning of the thirdzone and the beginning of the second zone.
 14. The method of claim 11further comprising: cutting in the third zone using a third commandspeed establishing a third torch speed; and initiating a ramp down ofthe cutting current at a location in the third zone determined by thethird torch speed and a time required for the current to reachsubstantially zero amperes.
 15. The method of claim 7 further comprisingcutting in the first zone or in the second zone using a gas flowcomposition comprising O2 plasma gas and O2 shield gas.
 16. A method forcutting a hole feature in a workpiece along at least a portion of a pathincluding a first zone, a second zone, and a third zone using a plasmacutting system, the method comprising: initiating a plasma gas flow;initiating a current flow to ignite a pilot arc; transferring the arc tothe workpiece; piercing the workpiece; cutting in a first zone and asecond zone, wherein a command speed of a cut for the second zone isdifferent than a command speed of a cut in the first zone; reducing acutting current in the third zone such that the cutting current reachessubstantially zero amperes at a point where an outer kerf edge of a cutin the third zone substantially meets an outer kerf edge of the cut inthe first zone; and decelerating a torch speed of the plasma cuttingsystem after the cutting current has reached substantially zero amperes.17. The method of claim 16 wherein a distance from a center of the holefeature to an outer kerf edge of the cut in the second zone issubstantially similar to a distance from the center of the hole featureto an outer kerf edge of the cut in the third zone at a point where thefirst and third zone intersect.
 18. The method of claim 16 furthercomprising decelerating the torch speed after a point where the outerkerf edge of the cut in the first zone substantially intersects with theouter kerf edge of the cut in the third zone.
 19. The method of claim 18wherein the torch speed reaches zero at a predetermined distance afterthe point where the outer kerf edge of the cut in the first zonesubstantially intersects with the outer kerf edge of the cut in thethird zone.
 20. The method of claim 16 wherein the first zonecorresponds to a lead-in of the cut, the second zone corresponds to aperimeter of the cut and the third zone corresponds to a kerf break-inregion of the cut.
 21. The method of claim 16 wherein the hole featureis substantially defined by an outer kerf edge of the cut in the secondzone and at least a portion of the outer kerf edge of the cut in thethird zone.
 22. The method of claim 16 further comprising cutting in thesecond zone with a command speed greater than the command speed of thecut in the first zone.
 23. The method of claim 16 wherein reducingcomprises ramping down the cutting current in the third zone such thatthe cutting current reaches substantially zero amperes at a locationwhere an outer kerf edge of the cut in the first zone substantiallyintersects with an outer kerf edge of the cut in the third zone.
 24. Amethod for cutting a hole feature in a workpiece along at least aportion of a path including a first zone, a second zone, and a thirdzone and using a plasma cutting system, the method comprising:initiating a plasma gas flow; initiating a current flow to ignite apilot arc; transferring the arc to the workpiece; piercing theworkpiece; cutting in the second zone with a command speed differentfrom a command speed of the first zone of the cut; ramping down acutting current in the third zone to remove a diminishing material suchthat an outer kerf edge of a cut in the third zone substantially alignswith an outer kerf edge of a cut in the second zone; and decelerating atorch speed of the plasma cutting system after the cutting current hasreached substantially zero amperes.
 25. The method of claim 24 furthercomprising ramping down the cutting current in the third zone so thatthe cutting current reaches substantially zero amperes where the outerkerf edge of the cut in the third zone intersects with the outer kerfedge of a cut in the first zone.
 26. The method of claim 24 wherein thediminishing material is defined at least in part by an outer kerf edgeof a cut in the first zone and an outer kerf edge of the cut in thethird zone.
 27. The method of claim 24 further comprising cutting in thethird zone with a command speed greater than the command speed of thesecond zone of the cut.
 28. A method for cutting a hole feature in aworkpiece using a plasma cutting system that reduces defects in the holefeature, the method comprising: initiating a plasma gas flow; initiatinga current flow to ignite a pilot arc; transferring the arc to theworkpiece; piercing the workpiece; establishing a cutting arc and a cutspeed with respect to the workpiece; increasing the cut speed to asecond cut speed after a first point in a hole cut path; ramping down acutting current after a second point in the hole cut path withoutreducing the cut speed; and substantially maintaining the second cutspeed until the cutting current reaches substantially zero amperes orincreasing the second cut speed to a third cut speed before the cuttingcurrent reaches substantially zero amperes.
 29. The method of claim 28wherein increasing comprises cutting in a first zone of the hole cutpath with a first command speed and cutting in a second zone of the holecut path with a second command speed greater than the first commandspeed.
 30. The method of claim 29 wherein the first zone defines alead-in of a cut and the second zone defines at least a portion of aperimeter of the hole feature.
 31. The method of claim 29 wherein thefirst command speed is based on a diameter of the hole feature.
 32. Themethod of claim 28 further comprising reducing the cutting current afterthe second point in the hole cut path and extinguishing the cutting arcsubstantially near the first point in the hole cut path.
 33. The methodof claim 28 further comprising: cutting from the second point in thehole cut path and returning back to the first point in the hole cut pathto form the hole feature in the workpiece; and ramping down the cuttingcurrent while cutting from the second point in the hole cut path back tothe first point in the hole cut path.
 34. A method for cutting a holefeature in a workpiece using a plasma arc torch to reduce defects in thehole feature and cutting along at least a portion of a path including afirst zone, a second zone and a third zone, the method comprising:selecting one of a plurality of cutting current ramp down operations forcutting in the third zone, wherein each of the plurality of cuttingcurrent ramp down operations is a function of a diameter of the holefeature; extinguishing the plasma cutting current when a torch headpasses from the third zone to the second zone at a location where thefirst zone, second zone and third zone substantially intersect; andsubstantially maintaining or increasing a torch speed in the third zoneuntil the torch head passes from the third zone into the second zone.35. A method for cutting a hole feature in a workpiece using a plasmaarc torch to reduce defects in the hole feature and cutting along atleast a portion of a path including a first zone, a second zone and athird zone, the method comprising: initiating a plasma gas flow;initiating a current flow to ignite a pilot arc; transferring the arc tothe workpiece; piercing the workpiece to begin cutting the hole feature;cutting along the first zone and the second zone of the path; initiatinga ramp down of a cutting current at a first point in the third zone suchthat the cutting current is extinguished at a second point where thefirst zone, the second zone and the third zone substantially intersect,the first point in the third zone determined based on a ramp down timeof the cutting current; and decelerating the plasma arc torch so that atorch speed reaches substantially zero at a predetermined distance afterthe second point.
 36. The method of claim 35 wherein the predetermineddistance is about ¼ of an inch.
 37. The method of claim 35 where in thepredetermined distance for a hole cutting speed of about 55 ipm and atable acceleration of about 5 mG is about ¼ of an inch.
 38. A method forcutting a hole feature in a workpiece along at least a portion of a pathincluding a first zone and a second zone using a plasma cutting system,the method comprising: initiating a plasma gas flow; initiating acurrent flow to ignite a pilot arc; transferring the arc to theworkpiece; piercing the workpiece; cutting in the first zone using afirst command speed establishing a first torch speed, the first commandspeed part of an acceleration curve; and cutting in the second zoneusing a second command speed establishing a second torch speed, thesecond command speed part of the acceleration curve and greater than thefirst command speed.
 39. A method for cutting a hole feature in aworkpiece along at least a portion of a path including a first zone anda second zone using a plasma cutting system, the method comprising:initiating a plasma gas flow; initiating a current flow to ignite apilot arc; transferring the arc to the workpiece; piercing theworkpiece; cutting where the first zone and the second zonesubstantially intersect at a first torch speed; and cutting at least aportion of the second zone at a second torch speed, the second torchspeed greater than the first torch speed.